(1) Field of the Invention
It is also an object of the present invention to provide a process for the manufacture of the copolymer solutions and their use as a binder component, the sheet-like structures which can be produced therefrom having, however, considerably improved properties in various respects. This includes the property that a lacquer which contains the new copolymer solutions in combination with organic polyisocyanates, when applied to a metallic substrate, after drying in the air for three to four days at temperatures of about 20.degree. C., should cure in such a way that the film no longer swells under the action of water and also cannot be removed mechanically by scratching, even if the film is exposed for a brief period to water at temperatures of 50.degree.-70.degree. C.
It is also a further object of the present invention that the copolymer solutions of the invention, in combination with other copolymer solutions which are built up, as thermoplastic copolymers, from a basis of lower alkyl methacrylates and acrylic acid or methacrylic acid, should make it possible to manufacture coating agents which dry at room temperature, by physical means, by solvent evaporation, to give films which have outstanding properties.
It is a further aim of the present invention to provide an acrylic resin which contains hydroxyl groups and which, in combination with polyisocyanates, provides the binder basis for air-drying lacquers and stoving lacquers which are suitable for top lacquering in the automobile industry, the lacquer being present after stoving in layer thicknesses of about 40 to 80 .mu.m and the top lacquer being located directly on top of the stoved primer of electrocoating lacquer.
In the modern methods of the present time for lacquering motor vehicles in an automobile factory, the coachwork parts are usually provided, after phosphating, with a primer of electrocoating lacquer and this is stoved to give a plastic coating. A spraying lacquer or a coating composition, which is generally designed a filler, is then applied to this plastic coating, usually by a spraying process, and stoving is carried out again.
The automobile lacquer formulation which contains pigment is then applied, in the desired pigmentation, to this two-layer coat of lacquer or primer by spraying and the automobile body is stoved again.
The whole coating, which, as explained above, consists of three layers, has a layer thickness after stoving of 70-100 .mu.m, the fraction relating to the outer layer of top lacquer being 30 to 40 .mu.m. In the course of rationalisation methods in the automobile industry efforts are being made to change over from the three-layer system to a two-layer build-up. In this, the electrocoat lacquering or another equivalent primer is intended to form the first coating layer and thereafter the final coat of lacquer is intended to be built up by means of the pigmented lacquer, but the total thickness of the layers applied is intended to have, in total, the same layer thickness. Using the conventional automobile top lacquers, however, it is not possible, in the customary spraying and stoving treatment, to produce, from lacquer formations containing pigment, coatings which, after drying in the air or after stoving, give lacquer films which are completely uniform (that is to say free from so-called curtaining and free from craters and blisters).
It is the aim of the present invention to provide an acrylic resin which contains hydroxyl groups and which, in combination with polyisocyanates, provides the binder basis for air-drying lacquers and stoving lacquers which are suitable for top lacquering in the automobile industry, the lacquer being present, after stoving, in layer thicknesses of about 40 to 80 .mu.m and the top lacquer being located directly on top of the stoved primer of electrocoating lacquer. In addition, the copolymers according to the invention, and also the coating/binder combinations consisting of the copolymers according to the invention and the polyisocyanates, should be miscible and compatible with thermoplastic copolymers which are soluble in organic solvents. The use of the theromplastic copolymers makes possible rapid drying, especially when used as a repair lacquering in order to touch up defective coats of lacquer. In such cases tack-free drying at room temperature within 5 to 10 minutes is required and the desired resistance to premium-grade motor fuels is met by means of the new binder combinations after a drying time of only one hour.
A further particular advantage resides in the fact that, for example, it is also possible partially to touch up after final assembly a lacquered automobile body on which painting defects still need to be touched up, if the reactive lacquer applied consists of a binder combination of the copolymers manufactured according to the invention and polyisocyanates. The touching up can be carried out on the partially or completely cross-linked binder combinations. With the binder combinations which are envisaged as a touching-up lacquer and which consist of 20 to 80% by weight, preferably 40 to 60% by weight of copolymers manufactured according to the invention and 80 to 20% by weight, preferably 60 to 40% by weight of thermoplastic copolymers, no differences in colour shade are found between the partially or completely cross-linked reactive lacquer and the touching-up lacquer, provided that the same pigments are employed, in terms of quantity and type.
In addition, the copolymer solutions according to the invention, which have a lower viscosity, exhibit a better absorption of pigment both when using inorganic pigments, such as, for example, titanium dioxide, or when using organic pigments, such as, for example, carbon black. When titanium dioxide is used as the pigment, the copolymers manufactured according to the invention, and also the reactive lacquer manufactured therefrom, still give high-gloss films at a pigment/binder ratio of 1.5 to 2:1, while the known copolymers and the known reactive lacquers manufactured therefrom show a distinct falling-off in gloss at the same level of pigmentation. In addition, it has been found that the copolymers manufactured according to the invention and the reactive lacquers manufactured therefrom produce high-gloss lacquers with carbon black, while the known copolymers and the reactive lacquers manufactured therefrom exhibit matt films at the same level of pigmentation.
It is a further object of the invention to provide a process for the manufacture of soluble organic copolymers which are produced as a solution having a solids content of 70 to 90% by weight, the copolymer solutions which have been formulated ready for spraying having at 25.degree. C. a viscosity of 25 seconds, measured in a DIN cup with a four mm flow orifice, and a solids conten of 40 to 65% by weight, preferably 45 to 65% by weight. The copolymer solutions manufactured according to the invention thus take account of the demands made by environmental protection for binders of high solids content and low solvent content.
The outstanding advantage of the copolymer solutions manufactured according to the invention resides in the fact that it is possible for the hydroxyl groups of the copolymers to crosslink with polyisocyanates at room temperature and also at elevated temperatures and to produce lacquers of high solids content, in a condition formulated ready for spraying, at a viscosity of 40 seconds, measured at 25.degree. C. in a DIN cup having a four mm flow orifice, and at a solids content of 61 to 80% by weight, preferably 65 to 80% by weight in the clear lacquer.
(2) Prior Art
Numerous proposals have been disclosed to manufacture solvent-resistant and alkali-resistant lacquers by reacting polyisocyanates and copolymers containing hydroxyl groups and to convert them into coatings. DT-AS No. 1,247,006 describes a process for the manufacture by the polyisocyanate polyaddition process of alkali-resistant sheet-like structures which are obtained from polyisocyanates and copolymers containing hydroxyl groups, but which are not adequately water-resistant after a reaction time of three to four days at 20.degree. C. These coatings obtained by this known process are, therefore, not suitable as top lacquers for external lacquerings which are resistant to weathering, since blistering takes place after a very short time and he adhesion of the lacquer to the various metallic substrates falls off and, as a result, the lacquer comes away from the substrate.
It is also known that polyhydroxyl compounds of polyols can be cured with polyisocyanates in order to obtain crosslinked lacquer coatings with good resistance properties. These combinations also exhibit too low a resistance to water and a lower resistance to weathering of the cured lacquer films.
French Pat. No. 1,556,309 describes a coating agent which contains solvent and, as the film-forming constituents, A. a copolymer consisting of (1) 2-50 parts by weight of an addition product with a 1:1 ratio of a carboxyl group of an .alpha.,.beta.-ethylenically unsaturated acid and an epoxide group of a glycidyl ester of an aliphatic carboxylic acid having tertiary C atoms, in which the aliphatic tertiary group contains 4-26 C. atoms, an (2) 98-50 parts by weight of unsaturated, copolymerisable monomers, the total quantity of (1) and (2) adding up to 100 parts by weight, and B. at least one polyisocyanate which contains at least 2 isocyanate groups in its molecule, the polyisocyanate being present, relative to the copolymer, in a quantity of 0.2-5 equivalents relative to the hydroxyl groups of the copolymer.
As the Examples in this French Patent Specification show, copolymer solutions are obtained which have a solids content of only 48 to 52%. The content of hydroxyl groups in Examples 1 to 10 is 0.8 to 3.18% by weight in the known copolymers.
In Example 1=0.8% of hydroxyl groups PA0 in Example 2=1% of hydroxyl groups PA0 in Example 3=1.26% of hydroxyl groups PA0 in Example 4=1.52% of hydroxyl groups PA0 in Example 5=1.41% of hydroxyl groups PA0 in Example 6=1.41% of hydroxyl groups PA0 in Example 7=1.66% of hydroxyl groups PA0 in Example 8=1.85% of hydroxyl groups PA0 in Example 9=1.85% of hydroxyl groups PA0 in Example 10=3.18% of hydroxyl groups.
If the procedure indicated in French Pat. No. 1,556,309 is then followed, the hydroxyl group content in the copolymers is increased, the viscosity is also increased. If the viscosity is reduced by dilution with xylene to 25 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice, the solids content of the copolymer solutions also decreases.
It cannot, however, be inferred from French Pat. No. 1,556,309 that it is possible, by selecting certain solvents and a certain range of quantities of a combination of various polymerisable monomers and using a combination of initiators, to manufacture copolymer solutions which have a substantially higher solids content and which open up the use of the new copolymers as binders for lacquer systems which are based on acrylates and have a low solvent content.
The particular advantage of the copolymer solutions manufactured according to the invention resides in the fact that the viscosity decreases when the hydroxyl group content in the copolymers is increased. When diluted with xylene to a viscosity of 25 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice, the solids content of the copolymer solutions manufactured according to the invention is then increased.
It has been found, surprisingly, that the copolymers which have been manufactured according to the invention and which have a hydroxyl group content of 3.5 to 6.5, preferably 4.5 to 5.5, %, lead to lower viscosities than do the known copolymers of the same hydroxyl group content. Thus, when dissolved to form a 70% strength by weight solution in ethylglycol acetate, the copolymer solutions manufactured according to the invention have viscosities of U to Z.sub.4, preferably X to Z.sub.3, measured on the Gardner-Holdt scale at 20.degree. C. Proof of this statement is afforded by a comparison between the copolymer solution manufactured according to the invention in Example 1 and the known comparison copolymers 3 and 4. As 70% strength by weight solutions in ethylglycol acetate, the comparison copolymers 3 and 4 lead to higher viscosities which are higher than Z.sub.6, measured on the Gardner-Holdt scale at 20.degree. C.
Compared with the known copolymers, the copolymers manufactured according to the invention display the following advantages in the combination with polyisocyanates: better solvent-resistance, higher abrasion resistance and better resistance to weathering in the Florida climate. In addition, in the combination with polyisocyanates, the copolymers manufactured according to the invention produce lacquers of a higher solids content, having a solids content of 61 to 80% by weight, preferably of 65 to 80% by weight, in the clear lacquer, when diluted with acetone, xylene or butyl acetate to a viscosity of 40 seconds at 25.degree. C., measured in a DIN cup having a 4 mm flow orifice. When such clear lacquers or pigmented lacquers are applied by spraying to steel sheets and are exposed to the air for a short time and subsequently stoved, high dry film layer thicknesses of 70 to 80 .mu.m are obtained, which do not exhibit crater formation or blistering. The known lacquer combinations which are described in French Pat. No. 1,556,309, only produce a solids content of 10 to 60% by weight in the clear lacquer when diluted with acetone, butyl acetate or xylene to a viscosity of 40 seconds at 25.degree. C. When such clear lacquers or pigmented lacquers are applied by spraying to steel sheets and are exposed to the air for a short time and subsequently stoved, lower dry film thicknesses of 40 to 50 .mu.m are obtained, which in addition also exhibit crater formation and blistering.